Polariton mode optical switch with composite structure

ABSTRACT

Devices, methods, and techniques for frequency-dependent optical switching are provided. In one embodiment, a device includes a substrate, a first optical-field confining structure located on the substrate, a second optical-field confining structure located on the substrate, and a composite structure located between the first and second optical-field confining structures. The second optical-field confining structure may be spaced apart from the first optical-field confining structure. The composite structure may include an embedding structure with a surface to receive photons and multiple quantum structures located in the embedding structure.

TECHNICAL FIELD

The described technology generally relates to optical switches and, moreparticularly, to polariton mode optical switches.

BACKGROUND

Optical switches are commercially important for use in a wide range ofapplications. For example, multiple optical switches may be used inoptical information processing and optical communications.

SUMMARY

Devices, methods, and techniques for frequency-dependent opticalswitching are provided. In one embodiment, a device includes asubstrate, a first optical-field confining structure located on thesubstrate, a second optical-field confining structure located on thesubstrate, and a composite structure located between the first andsecond optical-field confining structures. The second optical-fieldconfining structure may be spaced apart from the first optical-fieldconfining structure. The composite structure may include an embeddingstructure with a surface to receive photons and multiple quantumstructures located in the embedding structure.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill become apparent by reference to the drawings and the followingdetailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a perspective view of an illustrative embodiment of anoptical switching device with quantum wires.

FIG. 2 illustrates an electric field between two elongated metalstructures of the optical switching device shown in FIG. 1

FIG. 3A shows a perspective view of an illustrative embodiment of anoptical switching device with quantum dots.

FIG. 3B shows a cross-sectional view of the optical switching deviceshown in FIG. 3A taken along lines A-A′.

FIG. 4 shows a perspective view of an illustrative embodiment of anoptical switching device with wedge-shaped metal structures.

FIG. 5 shows a perspective view of an illustrative embodiment of anoptical switching device with photonic crystals.

FIG. 6 shows a perspective view of an illustrative embodiment of anoptical switching device with multiple composite structures.

FIG. 7 shows a flow chart of an illustrative embodiment of a method forfabricating an optical switching device.

FIG. 8 shows a flow chart of an illustrative embodiment of a method forfabricating an optical switching unit with quantum wires.

FIGS. 9A-9E are a series of diagrams illustrating the method shown inFIG. 8.

FIG. 10 shows a flow chart of another illustrative embodiment of amethod for fabricating an optical switching unit with quantum wires.

FIGS. 11A-11D are a series of diagrams illustrating the method shown inFIG. 10.

FIG. 12 shows a flow chart of an illustrative embodiment of a method forfabricating an optical switching device with quantum dots.

FIGS. 13A-13C are a series of diagrams illustrating the method shown inFIG. 12.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof In the drawings, similarsymbols typically identify similar components, unless context dictatesotherwise. The illustrative embodiments described in the detaileddescription, drawings, and claims are not meant to be limiting. Otherembodiments may be utilized, and other changes may be made, withoutdeparting from the spirit or scope of the subject matter presentedherein. It will be readily understood that the aspects of the presentdisclosure, as generally described herein, and illustrated in theFigures, can be arranged, substituted, combined, separated, and designedin a wide variety of different configurations, all of which areexplicitly contemplated herein.

FIG. 1 shows a perspective view of an illustrative embodiment of anoptical switching device 100. As depicted, optical switching device 100may include a substrate 110, a buffer layer 120 (optional) that may belocated on substrate 110, and an optical switching unit 130 located onbuffer layer 120.

In one embodiment, substrate 110 may be made of sapphire. In otherembodiments, substrate 110 may be made of appropriate semiconductormaterials. Examples of such semiconductor materials include, but are notlimited to, silicon (Si), germanium (Ge), or gallium arsenide (GaAs).Buffer layer 120 may be made of a material that is substantiallylattice-matched to the underlying substrate 110 and/or the overlyingoptical switching unit 130. In one embodiment, buffer layer 120 may bemade of one or more materials lattice matched to substrate 110. Examplesof such materials include, but are not limited to, GaAs, InGaAs, orAlGaAs.

Optical switching unit 130 is configured to receive photons 10 through aportion thereof, determine the wavelength of the received photons 10,and if the received photons 10 are of a prescribed wavelength, outputthe received photons 10, for example, through another portion thereof Inone embodiment, optical switching unit 130 may include multipleoptical-field confining structures, such as optical-field confiningstructures 131 a and 131 b (hereinafter collectively referred to asoptical-field confining structures 131), which are spaced apart fromeach other by a prescribed distance, and a composite structure, such asa composite structure 132, which is located between optical-fieldconfining structures 131. As used herein, the term “composite structure”refers to a structure including an embedding structure and one or morequantum structures embedded therein as reinforcing structures. Examplesof quantum structures include, but are not limited to, a quantum wire ora quantum dot. In one embodiment, as shown in FIG. 1, compositestructure 132 may include an embedding structure 136 and multiplequantum wires 137 embedded therein.

In one embodiment, optical switching unit 130 may receive photons 10through a surface of composite structure 132 (hereinafter referred to as“photon input surface”) and, if photons 10 are of a prescribedwavelength, output them (i.e., photons 10 of the prescribed wavelength)through another surface of composite structure 132 (hereinafter referredto as “photon output surface”). If photons 10 are not of the prescribedwavelength, optical switching unit 130 does not transmit the receivedphotons 10 that are not of the prescribed wavelength.

The physical operation of optical switching unit 130 may bequalitatively described as follows. As photons 10 are received throughthe photon input surface and travel through optical switching unit 130along the x-axis shown in FIG. 1, optical fields may be produced incomposite structure 132. These optical fields may lead to coupling ofelectrons in quantum wires 137 of composite structure 132 with photons10 travelling therethrough, to produce polaritons in quantum wires 137.For ease of explanation, the mode of operation of an optical switchingunit in which the polaritons are induced in quantum structures (for theembodiment shown in FIG. 1, quantum wires 137) will be referred to as a“polariton mode.”

Describing the polariton mode in more detail, one of quantum wires 137of optical switching unit 130 may have a strongly interactingBose-Hubbard Hamiltonian characteristic as represented by Equation 1below.

$\begin{matrix}{H = {{\sum\limits_{i}H_{i}^{JC}} - {\sum\limits_{i,j}{K_{ij}a_{i}^{\dagger}a_{j}}} - {\sum\limits_{i}{\mu_{i}N_{i}}}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

where i and j are integers, H_(i) ^(JC) is a Jaynes-Cummings Hamiltonianfor the photons and electron(s) at the i^(th) quantum structure, K_(ij)is the coupling constant between the i^(th) and j^(th) quantumstructures, μ_(i) is the chemical potential of the i^(th) quantumstructure, N_(i) is the total number of atomic and photonic excitationsin the i^(th) quantum structure, and α_(i) ^(†) is the creation operatorof the photon at the i^(th) quantum structure.

The Jaynes-Cummings Hamiltonian in Equation 1 may be expressed asEquation 2 below (the subscript “i” of H_(i) ^(JC) is omitted forsimplicity).

H ^(JC)=εσ₊σ⁻+ωα^(†)α+β(ω₊α+σ⁻α^(†))   [Equation 2]

where ε is the energy level difference between the electronic groundstate |g

and the excited state |e

of the i^(th) quantum structure, ω is the photon energy, β is theelectron-photon coupling constant, σ₊ and σ⁻ respectively are the atomicraising operator |e

| and the atomic lowering operator |g

e|.

Quantum wires 137 may have at least two discrete electron energy levelstates (e.g., a ground state and an excited state). The energy requiredto excite the electron in the lower electron energy level state to thehigher electron energy level state may be referred to as “transitionenergy” of a quantum wire. The difference between the transition energyand the energy of photons 10 of a specific wavelength may be referred toas “detuning Δ.” The detuning Δ may be expressed by Equation 3 below.

Δ=ω−ε  [Equation 3]

where ε is the energy level difference between the electronic groundstate |g

and the excited state |e

of a quantum structure, ω is the energy of the photon input to thequantum structure, and Δ is the detuning variable.

Strong optical fields in quantum wires 137 may lead to coupling of anelectron(s) in quantum wires 137 with the photon(s) passingtherethrough. This produces a dressed state (i.e., a combined state of nphotons with excited electron(s), or n polaritons, where n is a naturalnumber) in each of quantum wires 137, which is an eigenstate of theJaynes-Cummings Hamiltonian. Then polaritons may be described byEquation 4 below.

$\begin{matrix}{{{E_{n}^{\pm} = {{n\; \omega} - {{\Delta/2} \pm {\chi (n)}}}},{{\chi (n)} = \sqrt{{\beta^{2}n} + {\Delta^{2}/4}}}}{{{\pm n}\rangle} = \frac{\left\lbrack {{{- \left( {{\Delta/2} \mp {\chi (n)}} \right)}{{g,n}\rangle}} + {\beta \sqrt{n}{{e,{n - 1}}\rangle}}} \right\rbrack}{\sqrt{{2{\chi^{2}(n)}} \mp {\Delta \; {\chi (n)}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

where E_(n) ^(±) is the polariton energy of the n polaritons, n is thenumber of photons, ω is the photon energy, β is the electron-photoncoupling constant, Δ is the detuning variable, |±n

is the n-polaritons or the polariton state, |g,n

is the state in which the electron(s) are in the ground state with nphotons, and |e,n−1

s the state in which the electron(s) in the excited state are with n−1photons.

When in the polariton mode, optical switching unit 130, depending on thewavelength of photons 10, may selectively operate in either a superfluidmode or a mott insulator mode. The superfluid mode is a mode in whichphotons 10 in one of quantum wires 137 are allowed to hop to an adjacentquantum wire 137. The mott insulator mode is a mode in which photons 10in one of quantum wires 137 are not allowed to hop to an adjacentquantum wire 137. When in the superfluid mode, since photons 10 areallowed to hop through quantum wires 137, optical switching unit 130 maytransmit the received photons 10. In contrast, when in the mottinsulator mode, since photons 10 are not allowed to hop through quantumwires 137, optical switching unit 130 does not transmit received photons10.

The transition between the photon-blockade mode and the superfluid modeis determined by the order parameter Ψ=<α_(i)>=

n±|α_(i)|n±

When the angular frequency of photons 10 match that of the transitionenergy or the electron energy level separation of quantum wires 137(i.e., when Δ is zero), Ψ is equal to or near zero. In this case, thetunneling of photons 10 through adjacent quantum wires 137 may dominateover the repulsive interactions between photons 10 in quantum wires 137,and optical switching unit 130 may operate in the superfluid mode. Forcases where A is not zero, Ψ is not equal to or near zero. In this case,the repulsive interactions between photons 10 may dominate over thetunneling in quantum wires 137, and optical switching unit 130 mayoperate in the mott-insulator mode.

Quantum wires 137 may be made of one or more semiconductor materials,which may be selected based on the wavelength of the photons that are tobe transmitted by optical switching unit 130. In one embodiment, quantumwires 137 may be made of one or more materials selected from the groupconsisting essentially of Group I-VII semiconductor compounds, GroupII-VI semiconductor compounds and Group Ill-V semiconductor compounds.For example, quantum wires 137 may be made of Cd_(x)Zn_(1−x)S, where thevalue of x is between about 0 and about 1.0. In this example, foroptical switching unit 130 that is to transmit the photons having awavelength of about 450 nm (i.e., blue spectrum light), quantum wires137 may be made of Cd_(x)Zn_(1−x)S, where the value of x is betweenabout 0.5 and about 1.0. In another example, quantum wires 137 may bemade of CdSe_(x)S_(1−x), where the value of x is between 0 and 1. Inthis example, for optical switching unit 130 that is to transmit thephotons having wavelength of about 520 nm (i.e., green spectrum light),quantum wires 137 may be made of CdSe_(x)S_(1−x), where the value of xis between about 0 and about 0.4. Further, for optical switching unit130 that is to transmit the photons having wavelength of about 633 nm(i.e., red spectrum light), quantum wires 137 may be made ofCdSe_(x)S_(1−x), where the value of x is between about 0.6 and about1.0.

Optical switching unit 130 may be controlled by using a control lightsignal. For example, a control light signal of a prescribed wavelengthmay be supplied in conjunction with a data light signal of anotherprescribed wavelength. If the combined energy of the control lightsignal and the data light signal is substantially equal to thetransition energy of quantum wires 137 (i.e., Δ is zero), then thecontrol light signal and the data light signal may pass through opticalswitching unit 130. If the combined energy of the control light signaland the data light signal is not substantially equal to the transitionenergy, then the control light signal and the data light signal may beblocked by optical switching unit 130. The wavelength of the controllight signal may be varied to selectively perform on/off switching onthe data light signal of a prescribed wavelength.

Optical switching unit 130 may function as a wavelength-selectiveoptical switch if the optical field of photons 10 travellingtherethrough are confined in quantum wires 137. This is because strongeroptical fields in quantum wires 137 may help optical switching unit 130to operate in the polariton mode and, thus, operate as awavelength-selective optical switch. Optical-field confining structures131 disposed at the opposite ends of composite structure 132 may assistto better confine the optical fields to quantum wires 137 of compositestructure 132.

In one embodiment, as shown in FIG. 1, optical-field confiningstructures 131 may include multiple elongated metal structures 131 a and131 b spaced apart by a prescribed distance. Hereinafter, optical-fieldconfining structures 131 will also be referred to as elongated metalstructures 131. Two elongated metal structures 131 can function as asurface plasmon waveguide. FIG. 2 illustrates an electric field betweentwo elongated metal structures 131 (e.g., optical-field confiningstructures 131 a and 131 b) of optical switching device 100 shown inFIG. 1. Referring to FIG. 2, the electric fields confined betweenelongated metal structures 131 a and 131 b shown in FIG. 1 may beexplained by Equation 5 below.

$\begin{matrix}{\frac{D_{x\_ quantum}}{D_{x\_ metal}} = {\frac{ɛ_{quantum}E_{x\_ quantum}}{ɛ_{metal}E_{x\_ metal}} = {{1\therefore\frac{E_{x\_ quantum}}{E_{x\_ metal}}} = \frac{ɛ_{metal}}{ɛ_{quantum}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

where D_(x) _(—) _(metal) is an electric displacement field in elongatedmetal structure 131 a or 131 b along the x-axis, D_(x) _(—) _(quantum)is an electric displacement field in composite structure 132 along thex-axis, E_(x) _(—) _(metal) is an electric field in elongated metalstructure 131 a or 131 b along the x-axis, E_(x) _(—) _(quantum) is anelectric field in composite structure 132 along the x-axis, andε_(metal) is the permittivity of elongated metal structure 131 a or 131b, and ε_(dielectric) is the permittivity of composite structure 132.

As can be seen in FIG. 2 and from Equation 5, the electric field ofphotons 10 confined between elongated metal structures 131 a and 131 b(i.e., in composite structure 132) is proportional to the ratio betweenthe permittivity of elongated metal structure 131 a or 131 b and thepermittivity of composite structure 132. Thus, the desired confinementof the electrical field may be obtained by selecting a material(s) ofappropriate permittivity for elongated metals structures 131 and/orcomposite structure 132.

In one embodiment, elongated metal structures 131 may include one ormore various kinds of metals. The permittivity of a metal is a functionof frequency and, thus, the metal-type used may depend on the frequencyor wavelength of photons 10 that are to be transmitted by opticalswitching unit 130. In one embodiment, elongated metal structures 131may include a metal with a proper permittivity for a specific spectrum(e.g., the red, green, or blue spectrum). For example, elongated metalstructures 131 configured to detect the photons in the blue spectrum mayinclude one or more metals having a maximum permittivity in the bluespectrum. Examples of such metals include, but are not limited to, Ag,Al, Au, Ni, or Ti.

Considering that the permittivity of a metal is generally much higherthan that of a dielectric material (e.g., the material of embeddingstructure 136 and quantum wires 137 of composite structure 132), thearrangement of elongated metal structures 131 may generally enable astrong confinement of the electric field of photons 10. This holds trueeven for the case where two adjacent elongated metal structures 131 arespaced apart by a distance smaller than the wavelength of photons 10. Inone embodiment, two adjacent elongated metal structures 131 may bespaced apart by a distance equal to or smaller than the wavelength ofthe photons that are to be transmitted by optical switching unit 130. Inanother embodiment, two adjacent elongated metal structures 131 may bespaced apart by a distance equal to or smaller than one quarter of thewavelength of the incident photons that are to be transmitted by opticalswitching unit 130. For example, in cases where the frequency of thephotons to be transmitted by optical switching unit 130 is about 1 μm,optical switching unit 130 may be fabricated in a manner such that twoelongated metal structures 131 are spaced apart by a distance smallerthan about 250 nm. In some embodiments, two elongated metal structures131 may be spaced apart by a distance that is in the range from about afew nanometers to a few hundred nanometers. Elongated metal structures131 illustrated in FIG. 1 are in contact with composite structure 132.In other embodiments, however, elongated metal structures 131 may bespaced apart from composite structure 132 such that the distance betweentwo elongated metal structures 131 is lager than the width of compositestructure 132.

In one embodiment, the width, the thickness, and the length of compositestructures 132 may respectively be in the range of a few to a few tensof nanometers, a few hundred nanometers to a few hundred micrometers,and a few hundred nanometers to a few tens of micrometers. Also, in oneembodiment, the diameter and the height of quantum wires 137 mayrespectively be in the range of a few nanometers and a few tens to a fewhundred nanometers.

Optical switching device 100 described in conjunction with FIG. 1employs quantum wires 137 as its quantum structures embedded incomposite structure 132. However, it should be appreciated that anoptical device in accordance with the present disclosure may includedifferent types of quantum structures. For example, the compositestructure may include multiple quantum dots. In this regard, FIG. 3Ashows a perspective view of an illustrative embodiment of an opticalswitching device 300 with quantum dots, and FIG. 3B shows across-sectional view of optical switching device 300 shown in FIG. 3Ataken along lines A-A′. Similar to optical switching device 100illustrated in FIG. 1, optical switching device 300 may include asubstrate 310 and an optical switching unit 330 located on substrate310. While not shown in FIGS. 3A and 3B, in some embodiments, opticalswitching device 300 may optionally further include a buffer layerdisposed between substrate 310 and optical switching unit 330. Opticalswitching unit 330 may include multiple elongated metal structures 331 aand 331 b (hereinafter collectively referred to as “elongated metalstructures 331”) and a composite structure 332. Composite structure 332may include an embedding structure 336 and multiple quantum dots 337embedded therein. In one embodiment, embedding structure 336 may be madeof one or more porous materials. In the above embodiment, multiplequantum dots 337 may be disposed in the pores of embedding structure336. Examples of such porous materials include, but are not limited to,zeolite, silica, diatomite earth, pearlite or mulite. In anotherembodiment, embedding structure 336 may be made of one or more polymermaterials. Examples of such polymer materials include, but are notlimited to, polyamide, ionomer, polycarbonate, polyurethane,polystyrene, polyethylene, or fluoropolymer. Quantum dots 337 may besubstantially spherical in shape with diameters ranging from about 1 nmto about 30 nm. In other embodiments, the diameter of quantum dots 337may range from about 1 nm to about 9 nm.

Optical switching devices 100 and 300 described in conjunction with FIG.1 and FIGS. 3A and 3B respectively employ elongated metal structures 131and 331 of a substantially rectangular shape as their optical-fieldconfining structures. However, it should be appreciated that an opticalswitching device in accordance with the present disclosure may includemetal structures of a different shape. For example, the optical-fieldconfining structures may include multiple elongated wedge-shaped metalstructures. In this regard, FIG. 4 shows a perspective view of anillustrative embodiment of an optical switching device 400 withelongated wedge-shaped metal structures 431 a and 431 b. Similar tooptical switching device 100 illustrated in FIG. 1, optical switchingdevice 400 may include a substrate 410, a buffer layer 420 located onsubstrate 410, and an optical switching unit 430 located on buffer layer420. Optical switching unit 430 may include multiple elongatedwedge-shaped metal structures 431 a and 431 b, and a composite structure432 with a trapezoidal cross-section disposed between elongatedwedge-shaped metal structures 431 a and 431 b.

Further, the optical-field confining structures of the presentdisclosure are not limited to the metal structures acting as a surfaceplasmon waveguide(s). In some embodiments, photonic crystals may beemployed as the optical-field confining structures. In this regard, FIG.5 shows a perspective view of an illustrative embodiment of an opticalswitching device 500 with such photonic crystals 531 a and 531 b.Similar to optical switching device 100 illustrated in FIG. 1, opticalswitching device 500 may include a substrate 510, a buffer layer 520located on substrate 510, and an optical switching unit 530 located onbuffer layer 520. Optical switching unit 530 may include multiplephotonic crystals 531 a and 531 b, and a composite structure 532disposed between photonic crystals 531 a and 531 b.

Photonic crystals 531 a and 531 b may include multiple dielectric ormetallo-dielectric nanostructures. In one embodiment, as illustrated inFIG. 5, photonic crystals 531 a and 531 b may be a dielectric structurewith a multiple number of holes 540 defined therein. In one embodiment,holes 540 may be spaced apart by the same prescribed distance. Thediameter of holes 540 may be in the nanometer range. In otherembodiments, however, other types of photonic crystals may be employed.For example, photonic crystals with multiple nanorods spaced apart bythe same distance may be used. The above arrangement of photoniccrystals 531 a and 531 b may effectively confine the optical field ofphotons therebetween.

Optical switching devices 100, 300, 400 and 500 described in conjunctionwith FIGS. 1, 3A, 3B, 4 and 5 include two optical-field confiningstructures and a composite structure disposed therebetween. However, itshould be appreciated that an optical switching device in accordancewith the present disclosure may include more than two optical-fieldconfining structures and more than one composite structure. In thisregard, FIG. 6 shows a perspective view of an illustrative embodiment ofan optical switching device 600 with multiple composite structures 632 aand 632 b. Similar to optical switching device 100 illustrated in FIG.1, optical switching device 600 may include a substrate 610, a bufferlayer 620 located on substrate 610, and an optical switching unit 630located on buffer layer 620. Optical switching unit 630 may includethree elongated metal structures 631 a-631 c, and two compositestructures 632 a and 632 b disposed between elongated metal structures631 a-631 c. Only two composite structures and three elongated metalstructures are shown in FIG. 6 for simplicity and one skilled in the artwill appreciate that there may be a different number of compositestructures and/or elongated metal structures.

Composite structures 632 a and 632 b may respectively receive photons 61and 62 through a surface thereof and, if photons 61 and 62 are of aprescribed wavelength, respectively output them (i.e., photons 61 and 62of the prescribed wavelength) through another surface of compositestructures 632 a and 632 b. If photons 61 are not of the prescribedwavelength, composite structures 632 a and 632 b do not transmit (i.e.,output) received photons 61 and 62. In one embodiment, compositestructures 632 a and 632 b may be configured to transmit photons of thesame wavelength. In another embodiment, composite structures 632 a and632 b may be configured to transmit photons of different wavelengths.For example, elongated metal structures 631 a-631 c and/or compositestructures 632 a and 632 b (e.g., the quantum structures in compositestructures 632 a and 632 b) may be made of different materials, suchthat composite structures 632 a and 632 b may transmit photons ofdifferent wavelengths.

In some cases, it may be desirable to prevent photons passing throughthe quantum structures of one composite structure 632 a or 632 b fromhopping over to the quantum structures of another composite structure632 b or 632 a. In one embodiment, optical switching unit 630 may beconfigured in a manner such that composite structures 632 a and 632 bare spaced apart by a distance equal to or greater than one quarter thewavelength(s) of the photons that are to be transmitted by compositestructures 632 a and 632 b.

The aforementioned optical switching devices may be fabricated in avariety of ways, some of which are explained hereafter. FIG. 7 shows aflow chart of an illustrative embodiment of a method for fabricating anoptical switching device. Referring to FIG. 7, in block 71 0, asubstrate is prepared for the formation of an optical switching unitthereon. In block 720, a buffer layer is formed on the substrate, and inblock 730, the optical switching unit is formed on the buffer layer. Theoptical switching unit may include multiple optical-field confiningstructures located on the substrate and spaced apart from each other,and one or more composite structures made of a dielectric materialdisposed between multiple optical-field confining structures.

The buffer layer and the optical switching unit may be formed on thesubstrate using any one or a combination of a variety of suitabledeposition, etching, and/or photolithographic techniques known in theart. Examples of suitable deposition techniques include, but are notlimited to, chemical depositions techniques (e.g., chemical vapordeposition (CVD), plasma enhanced CVD (PECVD)), physical depositiontechniques (e.g., physical vapor deposition (PVD), or other depositiontechniques (e.g., molecular beam epitaxy (MBE)). Examples of suitableetching techniques include, but are not limited to, wet etching,anisotropic etching, or plasma etching. Further, in some embodiments,the composite structure(s) of the optical switching unit may be formedby using one or any combination of a variety of suitable micro ornanofabrication techniques known in the art. Such techniques include,but are not limited to, spin-coating, drop casting, printing, stamping,or self-assembly.

The particular processes performed in forming the optical switching unitmay vary depending on the type and shape of the optical-field confiningstructures and/or the composite structure(s) to be fabricated.

FIG. 8 shows a flow chart of an illustrative embodiment of a method forfabricating an optical switching unit with quantum wires. FIGS. 9A-9Eare a series of diagrams illustrating the method shown in FIG. 8.Referring to FIG. 8, in block 810, a dielectric layer 925 is formed on abuffer layer 920 that is disposed on a substrate 910, as shown in FIG.9A. In one embodiment, dielectric layer 925 may be formed by depositingdielectric material on buffer layer 920. In block 820, as shown in FIG.9B, a first portion 926 of dielectric layer 925 is patterned to definetherein multiple holes 940. In block 830, as shown in FIG. 9C, multiplequantum wires 937 are respectively formed in multiple holes 940. Inblock 840, as shown in FIG. 9D, second portions 927 of dielectric layer925 are removed. In block 850, as shown in FIG. 9E, elongated metalstructures 931 a and 931 b are formed in place of the removed secondportions 927. The above forming, patterning, and removing processes maybe performed by using any of a variety of suitable masking and/oretching processes known in the art (e.g., photolithography and plasmaetching techniques).

The method described in conjunction with FIG. 8 fabricates an opticalswitching unit having elongated metal structures 931 a and 931 b as itsoptical-field confining structures. For cases where an optical switchingunit having photonic crystals as its optical-field confining structuresis to be fabricated, blocks 840 and 850 may be replaced with otheroperations. For example, second portions 927 of dielectric layer 925 maybe patterned to define therein a multiple number of holes, therebyallowing for the forming of photonic crystals in second portions 927 ofdielectric layer 925.

FIG. 10 shows a flow chart of another illustrative embodiment of amethod for fabricating an optical switching device with quantum wires.FIGS. 11A-11D are a series of diagrams illustrating the method shown inFIG. 10. Referring to FIG. 10, in block 1010, quantum wires 1137 areformed on a portion 1126 of a buffer layer 1120 that is disposed on asubstrate 1110, as shown in FIG. 11A. In one embodiment, quantum wires1137 may be formed by using any one or a combination of a variety ofknown catalytic techniques, which use a catalyst(s) of one material informing a quantum wire of a different material. Examples of suchtechniques include, but are not limited to, vapor-solid (VS) techniquesor vapor-liquid-solid (VLS) techniques. For example, nano-sized dropletsof catalyst material (e.g., Au, Fe, Co, Mo, Ga, Al, Ti, or Ni) may bedeposited on portion 1126 of buffer layer 1120 and a gas mixtureincluding one or more semiconductor material may be introduced to growquantum wires 1137 under the droplets of catalyst material.

In block 1020, as shown in FIG. 11B, a dielectric layer 1125 is formedon buffer layer 1120. In one embodiment, dielectric layer 1125 may beformed to a thickness to expose at least some upper surfaces of quantumwires 1137. In other embodiments, dielectric layer 1125 may be formed toa thickness to completely bury quantum wires 1137 therein. Dielectriclayer 1125 may be formed by using any one or a combination of a varietyof suitable deposition techniques known in the art. In block 1030, asshown in FIG. 11C, portions 1127 of dielectric layer 1125 are removed toform a composite structure 1132. In block 1040, as shown in FIG. 11D,elongated metal structures 1131 a and 1131 b are formed in place ofremoved portions 1127 of buffer layer 1120. The above removing andforming process may be performed by using any one or a combination of avariety of suitable masking and/or etching processes known in the art(e.g., photolithography or plasma etching techniques).

FIG. 12 shows a flow chart of an illustrative embodiment of a method forfabricating an optical switching unit with quantum dots in accordancewith an illustrative embodiment. FIGS. 13A-13C are a series of diagramsillustrating the method shown in FIG. 12. Referring to FIG. 12, in block1210, a metal layer 1328 is formed on a substrate 13 10, as shown inFIG. 13A. In block 1220, as shown in FIG. 13B, a portion of metal layer1328 is removed to define therein a trench 1329 and form two elongatedmetal structures 1331 a and 1331 b. The above forming and removingprocesses may be performed by employing any one or a combination of avariety of suitable masking, deposition and/or etching processes knownin the art (e.g., photolithography or plasma etching techniques).

In block 1230, as shown in FIG. 13C, a composite structure 1332, whichincludes an embedding structure 1336 and multiple quantum dots 1337embedded therein, are formed in trench 1329 (i.e., between two elongatedmetal structures 1331 a and 1331 b). Composite structure 1332 may beformed by using any one or a combination of a variety of suitable microor nanofabrication techniques known in the art. In one embodiment,composite structure 1332 may be formed by dispersing pre-fabricatedquantum dots 1337 in a slurry or a solution including polymer materialsand applying the slurry or the solution on the exposed portions ofsubstrate 1310 as a film by spin-coating, drop casting, printing, doctorblade coating, or extrusion coating. The applied film may be driedand/or heated to form composite structure 1332. In another embodiment,composite structure 1332 may be formed by using a self-assembly process.The aforementioned composite structure forming processes are well knownin the art and can be accomplished without the need of furtherexplanation herein.

The method described in conjunction with FIG. 12 first forms elongatedmetal structures 1331 a and 1331 b and then composite structure 1332therebetween. However, it should be appreciated that in otherembodiments, composite structure 1332 may first be formed on substrate1310 and elongated metal structures 1331 a and 1331 b may be formedthereafter. For example, a thin film may be applied on substrate 1310and the outer portions of the thin film may be removed, thereby formingcomposite structure 1332. Thereafter, elongated metal structures 1331 aand 1331 b may be formed in place of the removed outer portions.

One skilled in the art will appreciate that, for this and otherprocesses and methods disclosed herein, the functions performed in theprocesses and methods may be implemented in differing order.Furthermore, the outlined steps and operations are only provided asexamples, and some of the steps and operations may be optional, combinedinto fewer steps and operations, or expanded into additional steps andoperations without detracting from the essence of the disclosedembodiments.

The present disclosure is not to be limited in terms of the particularembodiments described in this application, which are intended asillustrations of various aspects. Many modifications and variations canbe made without departing from its spirit and scope, as will be apparentto those skilled in the art. Functionally equivalent methods andapparatuses within the scope of the disclosure, in addition to thoseenumerated herein, will be apparent to those skilled in the art from theforegoing descriptions. Such modifications and variations are intendedto fall within the scope of the appended claims. The present disclosureis to be limited only by the terms of the appended claims, along withthe full scope of equivalents to which such claims are entitled. It isto be understood that this disclosure is not limited to particularmethods, reagents, compounds compositions or biological systems, whichcan, of course, vary. It is also to be understood that the terminologyused herein is for the purpose of describing particular embodimentsonly, and is not intended to be limiting.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

It will be understood by those within the art that, in general, termsused herein, and especially in the appended claims (e.g. bodies of theappended claims) are generally intended as “open” terms (e.g. the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes but is not limited to,”etc.). It will be further understood by those within the art that if aspecific number of an introduced claim recitation is intended, such anintent will be explicitly recited in the claim, and in the absence ofsuch recitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to embodiments containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g. “a” and/or “an” should be interpreted to mean “at least one”or “one or more”); the same holds true for the use of definite articlesused to introduce claim recitations. In addition, even if a specificnumber of an introduced claim recitation is explicitly recited, thoseskilled in the art will recognize that such recitation should beinterpreted to mean at least the recited number (e.g. the barerecitation of “two recitations,” without other modifiers, means at leasttwo recitations, or two or more recitations). Furthermore, in thoseinstances where a convention analogous to “at least one of A, B, and C,etc.” is used, in general such a construction is intended in the senseone having skill in the art would understand the convention (e.g. “asystem having at least one of A, B, and C” would include but not belimited to systems that have A alone, B alone, C alone, A and Btogether, A and C together, B and C together, and/or A, B, and Ctogether, etc.). In those instances where a convention analogous to “atleast one of A, B, or C, etc.” is used, in general such a constructionis intended in the sense one having skill in the art would understandthe convention (e.g. “a system having at least one of A, B, or C” wouldinclude but not be limited to systems that have A alone, B alone, Calone, A and B together, A and C together, B and C together, and/or A,B, and C together, etc.). It will be further understood by those withinthe art that virtually any disjunctive word and/or phrase presenting twoor more alternative terms, whether in the description, claims, ordrawings, should be understood to contemplate the possibilities ofincluding one of the terms, either of the terms, or both terms. Forexample, the phrase “A or B” will be understood to include thepossibilities of “A” or “B” or “A and B.”

In addition, where features or aspects of the disclosure are describedin terms of Markush groups, those skilled in the art will recognize thatthe disclosure is also thereby described in terms of any individualmember or subgroup of members of the Markush group.

As will be understood by one skilled in the art, for any and allpurposes, such as in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” and the like include the number recited andrefer to ranges which can be subsequently broken down into subranges asdiscussed above. Finally, as will be understood by one skilled in theart, a range includes each individual member. Thus, for example, a grouphaving 1-3 cells refers to groups having 1, 2, or 3 cells. Similarly, agroup having 1-5 cells refers to groups having 1, 2, 3, 4, or 5 cells,and so forth.

From the foregoing, it will be appreciated that various embodiments ofthe present disclosure have been described herein for purposes ofillustration, and that various modifications may be made withoutdeparting from the scope and spirit of the present disclosure.Accordingly, the various embodiments disclosed herein are not intendedto be limiting, with the true scope and spirit being indicated by thefollowing claims.

1. A device comprising: a substrate; a first optical-field confiningstructure located on the substrate; a second optical-field confiningstructure located on the substrate, the second optical-field confiningstructure spaced apart from the first optical-field confining structure;and a composite structure located between the first and secondoptical-field confining structures, the composite structure comprisingan embedding structure with a surface to receive photons and a pluralityof quantum structures located in the embedding structure.
 2. The deviceof claim 1, wherein the first and the second optical-field confiningstructures are configured to substantially confine therebetween anoptical field of the photons, so as to enable the quantum structures toselectively operate in a mott insulator mode and a superfluid modedepending on a wavelength of the received photons.
 3. The device ofclaim 1, wherein at least one of the first and the second optical-fieldconfining structures is in contact with the composite structure.
 4. Thedevice of claim 1, wherein at least one of the first and the secondoptical-field confining structures is spaced apart from the compositestructure.
 5. The device of claim 1, wherein the first and the secondoptical-field confining structures are spaced apart by a distance equalto or less than a wavelength of the photons.
 6. The device of claim 5,wherein the first and the second optical-field confining structures arespaced apart by a distance equal to or less than one quarter of thewavelength of the photons.
 7. The device of claim 1, wherein the firstand second optical-field confining structures are rectangular metalstructures.
 8. The device of claim 1, wherein the first and secondoptical-field confining structures are wedge-shaped metal structures. 9.The device of claim 1, wherein the first and second optical-fieldconfining structures are made of at least one material selected from thegroup consisting essentially of Ag, Al, Au, Ni, and Ti.
 10. The deviceof claim 1, wherein the first and the second optical-field confiningstructures are photonic crystals.
 11. The device of claim 1, wherein thequantum structures are made of one or more materials selected from thegroup consisting essentially of Group I-VII semiconductor compounds,Group II-VI semiconductor compounds, and Group Ill-V semiconductorcompounds.
 12. The device of claim 1, wherein the quantum structures aremade of Cd_(x)Zn_(1−x)S, where the value of x is between 0.5 and 1.0.13. The device of claim 1, wherein the quantum structures are made ofCdSe_(x)S_(1−x), where the value of x is between 0 and
 1. 14. The deviceof claim 1, further comprising a buffer layer disposed between thesubstrate and the first and the second optical-field confiningstructures.
 15. The device of claim 1, further comprising: a thirdoptical-field confining structure located on the substrate, the thirdoptical-field confining structure spaced apart from the first and secondoptical-field confining structures; and a second composite structurelocated between the second and third optical-field confining structures,the second composite structure comprising a second embedding structurewith a second surface to receive second photons and a plurality ofsecond quantum structures located in the second embedding structure. 16.The device of claim 15, wherein the composite structure and the secondcomposite structure are spaced apart by a distance greater than aprescribed value, the prescribed value being the greater of a wavelengthof the photons and a wavelength of the second photons.
 17. A method forfabricating a device, the method comprising: forming a first and asecond optical-field confining structure on a substrate, the first andsecond optical-field confining structures being spaced apart from eachother; and forming a composite structure between the first and thesecond optical-field confining structures, the composite structuresincluding an embedding structure with a surface to receive photons and aplurality of quantum structures located in the embedding structure. 18.The method of claim 17, wherein forming the composite structurecomprises: forming a dielectric layer on the substrate; patterning oneor more first portions of the dielectric layer to define therein aplurality of holes; and respectively forming the plurality of quantumstructures in the plurality of holes, and wherein forming the first andthe second optical-field confining structures comprises: removing secondportions of the dielectric layer; and respectively forming the first andthe second optical-field confining structures in place of the removedsecond portions.
 19. The method of claim 17, wherein forming the firstand the second optical-field confining structures comprises: forming ametal layer on the substrate; and patterning the metal layer to form thefirst and the second optical-field structures, and wherein forming thecomposite structure comprises: forming the composite structure betweenthe first and the second optical-field structures.
 20. The method ofclaim 17, wherein the first and the second optical-field confiningstructures are spaced apart by a distance equal to or less than awavelength of photons that are to be transmitted by the device.
 21. Themethod of claim 20, wherein the first and the second optical-fieldconfining structures are spaced apart by a distance equal to or lessthan one quarter of a wavelength of the photons that are to betransmitted by the device.
 22. The method of claim 17, wherein the firstand the second optical-field confining structures are metal structures.23. The method of claim 17, wherein the first and the secondoptical-field confining structures are made of a material selected formthe group consisting of Ag, Al, Au, Ni, and Ti.
 24. The method of claim17, wherein the first and the second optical-field confining structuresare photonic crystals.